1. Field of the Invention
The present invention relates to a combined cycle power plant, and more particularly to a combined cycle power plant composed of a gas turbine system, a steam turbine system and a heat recovery steam generator.
2. Description of the Related Art
For recent combined cycle power plants, the development for the higher output and the higher heat efficiency is progressed, and the problem with respect to the combined cycle power plants when the combustion gas temperature at the inlet of a gas turbine is raised to 1500xc2x0 C. class from 1300xc2x0 C. class is under the verification.
When the combustion gas temperature at the gas turbine inlet is raised up to 1500xc2x0 C. class, it is not possible to sufficiently cool down rotor blades and stator blades which are gas turbine high temperature portions by a so-called air cooling system to cool the high temperature portions by the compressed air extracted from an air compressor.
So, it has become to select steam that has larger specific heat than air and is suited for the heat absorption as a new cooling medium.
On the other hand, as the steam fed to the steam turbine from the heat recovery steam generator rises to a high temperature with the increase in the combustion gas temperature of the gas turbine plant, it becomes necessary to control the temperature of the high pressure steam outputted from the heat recovery steam generator to a more proper temperature than before. If the temperature of the high pressure steam outputted from the heat recovery steam generator becomes abnormally high, it exceeds a maximum allowable temperature of the steam turbine plant, and the excessive thermal stress is generated. As a result, it becomes difficult to maintain the material strength of its piping and equipments (various valves, steam turbines, etc.) and they can be deformed and damaged.
Furthermore, if the temperature of the steam to cool down the high temperature portions of the gas turbine is too high, the high temperature portions of the gas turbine cannot be sufficiently cooled, and it exceeds critical temperatures of the materials composing the gas turbine. As a result, such serious troubles of the gas turbine as deformation, damage, etc. may be caused.
Therefore, in the prior art disclosed in the Japanese Patent Disclosure (Kokai) No. Hei 9-112292, a combined cycle power plant is examined with the construction as described below. That is, in the combined cycle power plant, after the saturated steam outputted from a high pressure drum is superheated by a first high pressure superheater to become a superheated steam, this superheated steam is branched into two steams at the outside of a casing of a heat recovery steam generator. One of them (for convenience, this steam is called as a main stream side steam) is introduced into a second high pressure superheater via a first control valve, and its steam temperature is further raised by the gas turbine exhaust gas. The other steam (for convenience, this steam is called as a de-superheating steam) flows through a de-superheating steam bypass side steam piping installed at the outside of the casing of the heat recovery steam generator via a second control valve so as to bypass the second high pressure superheater. And then, the two steams in different temperatures are merged immediately before introduced into a high pressure steam turbine so that a temperature of the high pressure steam that is introduced into the high pressure steam turbine does not rise excessively.
However, according to the prior art disclosed in the Japanese Patent Disclosure (Kokai) No. Hei 9-112292, two problems shown below have not yet been solved.
(1) Blocking Protection Measure for Control Valves
In the prior art, no safety measure for the blocking protection is provided in the first and second control valves. If two control valves have no safety measure for blocking protection, the control valves may possibly be blocked by erroneous operation. If at least one of the control valves is blocked, the heat recovery steam generator may be damaged by the abnormal pressure rise, the tubes provided at the downstream of the control valves may be damaged as a result of burning without the steam, or the gas turbine may be damaged due to the insufficient cooling steam.
(2) Thermal Stress Protection Measure for Steam Merging Portion
Depending on the operating state of the plant, a large difference may be generated between the temperature of the main stream side steam after passing the superheater and that of the de-superheating steam. In this case, a large thermal stress may possibly be generated at the steam merging portion. If a large thermal stress is generated at this steam merging portion, the damage of the piping may result.
Accordingly, one object of the present invention is to provide a combined cycle power plant using steam as a de-superheating medium of superheated steam which can protect a heat recovery steam generator and a gas turbine from being broken and the piping from being damaged.
These and other objects of this invention can be achieved by providing a combined cycle power plant including a gas turbine plant with a gas turbine provided with a high temperature cooled portion, a heat recovery steam generator to generate steam by recovering thermal energy of exhaust gas from the gas turbine plant, a steam turbine plant driven by the steam generated from the heat recovery steam generator, and a cooling steam system to flow the steam from the heat recovery steam generator to the high temperature cooled portion of the gas turbine for cooling. The heat recovery steam generator includes a main stream side steam piping, a bypass side steam piping, a steam branching portion connected to the main stream side steam piping and the bypass side steam piping to branch a steam flowing from a former stage in the heat recovery steam generator into two steams, one as a main stream side steam flowing through the main stream side steam piping and another as a de-superheating steam flowing through the bypass side steam piping, a high pressure superheater connected to the main stream side steam piping for superheating the main stream side steam, and a steam merging portion to merge the main stream side steam superheated by the high pressure superheater and the de-superheating steam passed through the bypass side steam piping. The steam merged in the steam merging portion is supplied to a next stage in the heat recovery steam generator, and the heat recovery steam generator is provided with a blocking prevention function to prevent blocking of the main stream side steam piping and the bypass side steam piping and a thermal stress generation protection function.
According to one aspect of this invention, there is provided a combined cycle power plant including a gas turbine plant with a gas turbine provided with a high temperature cooled portion, a heat recovery steam generator to generate steam by recovering thermal energy of exhaust gas from the gas turbine plant, a steam turbine plant driven by the steam generated from the heat recovery steam generator, and a cooling steam system to flow the steam from the heat recovery steam generator to the high temperature cooled portion of the gas turbine for cooling. The heat recovery steam generator includes a main stream side steam piping, a bypass side steam piping, a steam branching portion connected to the main stream side steam piping and the bypass side steam piping to branch a steam flowing from a former stage in the heat recovery steam generator into two steams, one as a main stream side steam flowing through the main stream side steam piping and another as a de-superheating steam flowing through the bypass side steam piping, a high pressure superheater connected to the main stream side steam piping for superheating the main stream side steam, and a steam merging portion to merge the main stream side steam superheated by the high pressure superheater and the de-superheating steam passed through the bypass side steam piping. The steam merged in the steam merging portion is supplied to a next stage in the heat recovery steam generator. The steam merging portion includes a three way valve provided with a first inlet piping, a second inlet piping and an outlet piping. The first inlet piping is connected to the main stream side piping at an outlet of the high pressure superheater for taking in the main stream side steam, and the second inlet piping is connected to the bypass side steam piping for taking in the de-superheating steam. The three way valve merges the main stream side steam superheated by the high pressure superheater and the de-superheating steam passed through the bypass side steam piping, and the steam merged in the three way valve is supplied from the outlet piping to the next stage in the heat recovery steam generator. The heat recovery steam generator is provided with a blocking prevention function to prevent blocking of the main stream side steam piping and the bypass side steam piping.